The invention relates to a process for the determination of internal pressure, particularly of the minimum pressure of the tire of a motor vehicle during driving operation, through an analysis of the characteristic vibration behavior of the wheel, whereby the standardized amplitude is determined from the vibration spectrum detected and the amplitude maxima of the resonance frequency are observed.
One such process is known, for example, from the patent document U.S. Pat. No. 6,450,020 B1. The German patent document DE 101 57 858 A1 discloses a process for determining the air pressure of the tire from the vehicle operation at low and high speeds, which is prepared independently by various elements attached to the vehicle. This process is based on a resonance frequency extracted on the basis of a wheel speed signal issued by a speed sensor assigned to every wheel. The determination of the air pressure of the tire is, furthermore, based on a dynamic load radius derived from the wheel speed signal.
The intensity and frequency situation of the tire vibrations that arise during travel are highly dependent upon the internal pressure of the tire, which determines both the rigidity as well as the damping of the wheel. These characteristics also determine the intensity and frequency with which the rim is stimulated to rotational vibrations. The rotational vibrations of the wheel can also be measured with an ABS rotational speed sensor, for example.
The rotational vibrations appear most clearly at the resonance frequencies that are dependent on the internal pressure.
In order to operate a motor vehicle as economically as possible, and in order to achieve a high level of consistency, the tires must be filled with a preset air pressure. The air pressure drops rapidly because of losses of diffusion and density in the tires. The leakage rates that arise are unavoidable, and typically lie within the range of a few 100 mbar per year. The air pressure of the tire must therefore be monitored regularly. The average driver, however, tends to either give the tire pressure no attention, or at least not the appropriate level of attention. For this reason, motor vehicles are increasingly equipped with tire pressure monitoring systems that continuously monitor the air pressure in the tires and issue a warning to the driver if the tire pressure drops below a preset minimum pressure.
Direct measuring systems are known in which a sensor that measures the pressure is located within the pressure space of the vehicle wheel. The measuring system reading out the internal pressure of the tire that is determined is, as a general rule, supplied with power by a battery and transmits its measuring data over a spark gap. The disadvantage of this system is, in addition to the operability, which is dependent on the service life of the battery, also the unbalanced dimension on the rim, ranging up to 20 g, that comes about from the sensor and the power supply.
Indirectly measuring systems make use of the change in the degree of the rolling of a wheel that occurs upon changes in the air pressure in the tires. The change in the extent of rolling is determined by means of the ABS sensors that monitor all of the wheels of a vehicle simultaneously and evaluate the corresponding data. These systems have the fundamental disadvantage that a nearly identical pressure loss in all tires, such as necessarily occurs from leakage losses, can not be detected.
The Japanese patent document JP 5-133 831 discloses a device for determining the internal pressure of a vehicle tire. In this device, the resonance frequency of the tire is determined by a frequency analysis that is carried out in relation to a speed signal of the wheel, including the vibration frequency of the tire. The current pressure is determined from the resonance frequency detected. The resonance frequency used for the determination of the air pressure varies within the range of 30–50 Hz. This range is adequate if the vehicle is in a lower or medium speed range. If the speed of the vehicle increases, then the vibration behavior of a tire is difficult to evaluate. As the result, the spectrum of the resonance frequency drops, and thereby impairs its significance in regard to the internal tire pressure.
A device by means of which several resonance frequencies from several frequency ranges, such as a range from 30 to 50 Hz and a range from 60 to 90 Hz, for example, are determined, is described in JP 8-219 920.
The knowledge that the wheel speed signal of a wheel being driven contains a resonance frequency of a high order, which then changes very greatly upon a change in the internal pressure of the tire if the speed of the vehicle is high, forms the basis for the process in accordance with U.S. Pat. No. 6,385,553. In this publication, it is described in very detailed form how the frequency spectrum and the resonance frequency that are of interest are determined by mathematical means, such as by a Fourier transformation (FFT), for example, so that a standardized amplitude can be plotted over the frequency.